Girls, my son has developed an unexplained rash. We went to the infect…

Girls, my son has developed an unexplained rash. We went to the infectious clinic (to make sure it wasn't croup), they said it was allergies, and it hasn't gone away after three days. The doctor prescribed Zyrtec drops and Normobact L, but I still don't understand what caused him to get so itchy ? I'm wiping his chest with a cloth, but there's been no improvement yet, as if it got worse